The suprachiasmatric nuclei (SCN) of the mammalian hypothalamus contain a circadian pacemaker that drives a wide variety of physiological and behavioral rhythms. Photic information is responsible for entrainment to the environmental light/dark cycle. Until recently, there has been little information regarding the molecular basis of entrainment to light. Studies undertaken in this laboratory and others have shown that light induces c- fos (mRNA and protein) in the SCN of the golden hamster, with a circadian dependence. Importantly, since photic induction of c-fos is restricted to light sensitive circadian times, the clock mechanism must be regulating the expression of this pathway. The objectives of the proposed research are to define: (1) Neurotransmitter agonists capable of circadian-dependent induction of c-fos mRNA and AP-1 DNA binding activity in SCN explants, maintained in vitro. (2) Second messenger pathways capable of mediating the induction of c-fos mRNA and AP-1 DNA binding in SCN explants. (3) The level of signal transduction at which c-fos mRNA and AP-1 DNA binding induction is under circadian control. By determining the signal transduction step(s) at which the circadian clock is interacting with the c-fos and AP-1 induction pathways, our understanding of the cellular components that comprise the clock mechanism will be facilitated. Excitatory amino acid and cholinergic agonists will be applied to SCN explants at various circadian times (defined by extrapolation from the donor's running wheel activity record). The explants will then be measured for AP-1 DNA binding activity or c-fos mRNA by gel-shift analysis and RNA- PCR respectively. Pharmacological manipulation of the explants will be utilized at various circadian times to probe the pathway(s) leading to c- fos/AP-1 induction. Gel-shift analysis will be utilized to test for circadian variations in the levels of the transcription factors CREB and SRF, known to regulate transcription of c-fos in a variety of systems. The elucidation of mechanism underlying circadian rhythms may aid in the understanding and treatment of a number of mental health disorders.